We present a durability test of polymer electrolyte membrane fuel cell ͑PEMFC͒ in open circuit condition. Such a condition enhances the deterioration of the membrane electrode assembly. ac impedance spectroscopy measurements and scanning electron microscopy observation suggest that the degradation occurred at the cathode. Direct gas mass spectroscopy of the cathode outlet gas indicated the formation of HF, H 2 O 2 , CO 2 , SO, SO 2 , H 2 SO 2 , and H 2 SO 3 . A kinetic model is presented assuming that the H 2 gas cross leakage from the anode caused the cathode degradation. The model determines the rate of degradation using the permeability across the electrolyte membrane measured for crossover H 2 gas.Future demand for ecologically benign power generation and transportation has motivated studies on polymer electrolyte membrane fuel cell ͑PEMFC͒. A number of studies have recently focused on the durability characteristics of the membrane electrode assembly ͑MEA͒ and the impact of operational conditions. 1-6 It has been reported that the open circuit operation without electric loading enhances the degradation of the MEA with H 2 gas and air supply. [7][8][9] The gradual decrease of the open circuit ͑OC͒ voltage V OC over 100 h is followed by a sudden increase of H 2 gas leakage across the electrolyte membrane, which has been ascribed to the formation of radical hydroperoxyl HO 2 ͑Ref. 7, 8, and 10-12͒ or thermal decomposition. 9 This study aims to provide a model for the degradation mechanism under OC operation, which is thus to be used as a means for lifetime assessment.Gas chromatogram ͑GC͒ can be used to analyze the outlet exhaust gas to identify the gas components resulting from degradation processes. GC requires a column suitable to separate gases. For example, MS-5A column often used for H 2 gas determination cannot be applied for CO 2 gas determination because the latter is too adsorptive. Therefore, we used direct gas mass spectroscopy ͑DGMS͒ which can separate gases without using a column. ExperimentalCarbon papers 200 m thick, coated with particulate Pt catalysts of 0.5 mg/cm 2 ͑Electrochem Inc., MA͒ over 5 cm 2 active area, were hot-pressed for 10 min at 120°C and pressure of 0.7 N/cm 2 onto free-acid form of Nafion 112 which had been precleaned with 3 wt % aq. H 2 O 2 and 1 N aq. H 2 NO 3 at 60°C in order to remove organic and cationic impurities. The resulting MEA was enclosed in a serpentine cell ͑Electrochem Inc.͒.Durability testing was carried out with an electric load PLZ152WA ͑Kikusui, Japan͒. H 2 and O 2 gases were supplied whose temperatures were kept at 70°C in the bubble-through humidifier. The flow rate was set at 40 mL/min while we confirmed that it had no significant effect on the MEA degradation. Before and after a 24 h OC operation, impedance analysis was done with an LCR tester 3522LCR ͑Hioki, Japan͒. The specimens for scanning electron microscopy ͑JEOL͒ were dried and then collected from the cell using a single-edge razor blade ͑Kaijirushi, Japan͒.DGMS ͑Bruker AXS͒ was conducted to chara...

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Management of water is one of the most important issues that limits the operation of polymer electrolyte fuel cells ͑PEFCs͒. To better understand water transport in polymer electrolyte membranes ͑PEMs͒, a one-dimensional model has been analyzed for water transport in a PEM. To increase the model accuracy, magnetic resonance imaging ͑MRI͒ experiments were done to determine the optimum parameters for the model. Using the resulting parameters, we programmed the measured number of maximum water content of the membrane in a fuel cell and water transfer coefficient of the membrane surface. The resulting water distribution of the PEM under various operation conditions was consistent with previous MRI measurements.The polymer electrolyte fuel cell ͑PEFC͒ is considered to be a useful power source for automotive and other applications. However, the ionic conductivity of polymer electrolyte membranes ͑PEMs͒ is high only when PEMs are hydrated. 1-4 In addition, hydrated PEMs last much longer than dehydrated PEMs. 5,6 The common method to keep a PEM hydrated is to supply humid gas to the PEFC. However, too high a humidity causes flooding of the porous carbon electrode. Thus, it is important to know the water content distribution in a PEM under fuel cell operation.There are many approaches to modeling the water transport in a PEM. 7-10 A parametric study 9 showed that some modeling parameters change the simulation results. For this reason, we developed a method based on magnetic resonance imaging ͑MRI͒ to determine the water distribution in a membrane under various operating conditions. We found that the water in the membrane has a concentration gradient under a high current in Ref. 11.In this study, we numerically analyzed the water transport in the membrane based on the MRI measurements. We focus here on two parameters: the water transfer coefficient through the membrane and gas diffusion layer ͑GDL͒, and the maximum value of the water content of the membrane in the fuel cell.There are few reports on the water transfer coefficient. Berg et al. determined it by comparing a two-dimensional model with a measured current density distribution. 12 However, the water transfer coefficient strongly depends on the porosity and thickness of the GDL and catalyst layer, so we considered it important to measure the water transport coefficient of our fuel cell. We used MRI because, unlike other methods, the water content increase in the PEM can be measured directly.Concerning the maximum water content, it is known that isolated PEMs swell about 10-20% in thickness and the lateral direction when they are hydrated. 2,13 However, in a fuel cell the PEMs and GDLs are constrained, a phenomenon that has received little study. For this reason, the hydration of a PEM in a fuel cell is poorly understood. With MRI, we could determine the water content of a PEM in a fuel cell under various humidification conditions. In addition, the MRI was useful in determining the relation between water content and swell.We used the measured water transfer coeffici...

Mixed potential sensors for the detection of hydrocarbons and carbon-monoxide have been previously studied at Los Alamos National Laboratory ͑LANL͒. The LANL sensors have a unique design incorporating dense ceramic-pellet/metal-wire electrodes and porous electrolytes. When these sensors are exposed to various gases, in addition to their mixed-potential response, their resistance changes. This change in resistance is probably associated with the oxygen reduction reaction at the electrode/electrolyte interface and can be used to yield a total NO x response. The NO x sensors are operated in a current bias mode where the voltage response is related to the total NO x concentration.Mixed potential sensors have been studied for the detection of carbon monoxide, hydrocarbons and nitrogen oxides over the past three decades. 1-5 However, there are no commercial devices in the market based on this phenomenon. This is primarily due to the fact that the mixed-potential is dependant on electrochemical oxidation and reduction reaction rates and is therefore a strong function of the electrode, electrolyte, and electrode/electrolyte/gas three-phase interface morphologies. Over the past decade we have developed various mixed potential sensors for non-methane hydrocarbons 6,7 and carbon monoxide 8,9 sensing. These sensors have unique sensor designs with improved electrode, electrolyte and three-phase interface stability incorporating either metal wire or dense oxide electrodes in combination with a porous electrolyte. 7,9 The use of either metal wires or ceramic pellets as electrodes ensures that the morphology of these electrodes and the three-phase interface is both reproducible from sensor to sensor and is very stable over time.The signs of the mixed-potential response for NO and NO 2 are in opposite directions while using an oxygen ion conducting electrolyte because NO and NO 2 are reducing and oxidizing gases respectively. This is illustrated in Fig. 1 10 where NO, C 3 H 6 , C 3 H 8 , and CO yield positive responses while NO 2 yields a negative response. This is consistent with the fact that the hydrocarbons, CO and NO are electrochemically oxidized while NO 2 is electrochemically reduced at the sensing electrode/electrolyte interface. It is also seen in Fig. 1 that when the sensor is operated in combination with a catalyst ͑1% Pt on ␥-Al 2 O 3 ͒ the response to both NO and NO 2 is in the positive direction and hydrocarbon interference is mitigated. The catalyst converts the hydrocarbons to CO 2 and H 2 O while converting the NO 2 to NO ͑the stable species at 21% O 2 and 600°C͒. Because the sensor response is relatively insensitive to the presence of H 2 O and CO 2 , the primary response of the sensor is to NO and NO 2 . Therefore the sensor in combination with the catalyst could potentially be used as a total NO x sensor. However the sensor sensitivity is too low and the use of a catalyst operated at a temperature different from that of the sensor, complicates the sensor design. Therefore, it would be advantageous to eliminat...